1. Field of the Invention
This invention relates to improvements in processes for the catalytic hydrodewaxing of hydrocarbon chargestocks. More specifically, the present invention is concerned with restoring the activity of spent hydrodewaxing catalysts to fresh activity.
2. Description of the Prior Art
The cracking and/or hydrocracking of petroleum stocks is, in general, well known and widely practiced. It is known to use various zeolites to catalyze cracking and/or hydrocracking processes. The cracking may have the intent to convert a certain class of compounds in order to modify a characteristic of the whole petroleum stock. Exemplary or this type of conversion is shape selective conversion of straight and slightly branched aliphatic compounds of 12 or more carbon atoms to reduce pour point, viscosity, and/or to improve pumpability of heavy fractions which contain these waxy constituents. The long carbon chain compounds tend to crystallize on cooling of the oil to an extent such that the oil will not flow, hence, may not be able to be pumped, transported by pipelines, or useful for effective lubrication. The temperature at which such mixture will not flow is designated as the "pour point," as determined by standardized test procedures.
The pour point problem can be overcome by techniques known in the art for removal of waxes or conversion of these compounds to other hydrocarbons which do not crystallize at ambient temperatures. An important method for so converting waxy hydrocarbons is shape selective cracking or hydrocracking utilizing principles described in U.S. Pat. No. 3,140,322, dated July 7, 1964. Zeolite catalysts for selective conversions of wax described in the literature include such species as mordenite, with or without added metal to function as a hydrogenation catalyst.
Particularly effective catalysts for catalytic dewaxing include zeolite ZSM-5 and related porous crystalline aluminosilicate zeolites as described in U.S. Pat. No. Re. 28,398 (Chen et al), dated Apr. 22, 1975. As described in that patent, drastic reductions in pour point are achieved by catalytic shape selective conversion of the wax content of heavy stocks with hydrogen in the presence of a dual-function catalyst of a metal plus the hydrogen form of ZSM-5. The conversion of waxes is by scission of carbon-to-carbon bonds (cracking) and production of products of lower boiling point than the wax. However, only minor conversion occurs in dewaxing. For example, Chen et al describes hydrodewaxing of a full range shale oil having a pour point of +80.degree. F. to yield a pumpable product of pour point at -15.degree. F. The shift of materials from the fraction heavier than light fuel oil to lighter components was in the neighborhood of 9 percent conversion.
Catalytic hydrodewaxing of middle distillates and higher boiling lubricating oil stocks has been successfully developed to a stage of commercial operation. The process effectively dewaxes most of the distillate feeds available. However, individual chargestocks, of similar boiling point, may vary as far as the difficulty in catalytically dewaxing the respective chargestocks. This difficulty is manifested in the need for higher initial reaction temperatures for dewaxing, as much as 50.degree. F. It is speculated that much of the difference in reaction temperature is due to differences in the content of compounds which are poisonous to the dewaxing catalyst and contained in the chargestock. In particular, differences in nitrogen-containing compounds are suspected of being the primary reason certain chargestocks require higher reaction temperatures.
It is known to remove poisonous nitrogen-containing compounds from waxy feedstocks prior to catalytic dewaxing. Thus, it is known to pass the feed to be dewaxed initially through a bed of hydrotreating catalyst and thereafter through a bed of the hydrodewaxing catalyst. Such a process is described in U.S. Pat. No. 4,257,872, dated Mar. 24, 1981. The hydrotreating catalysts typically employed are metals or metal oxides of Group VIB and/or Group VIII deposited on a solid porous support such as silica and/or metal oxides such as alumina, titanium, zirconia, or mixtures thereof. Representatives of Group VIB metals include molybdenum, chromium and tungsten and Group VIII metals include nickel, cobalt, palladium and platinum. The metal components are deposited, in the form of metals or metal oxides, on the indicated supports in amounts generally between about 0.1 and about 20 wt.%. Initial hydrotreating of the feed serves to convert sulfur, nitrogen, and oxygen derivatives of the hydrocarbon feed to hydrogen sulfide, ammonia and water.
It is also known to increase the effectiveness of a dewaxing catalyst such that the dewaxing catalyst behaves as if it were catalytically more active or more resistant to aging by pretreating the hydrocarbon feed with a zeolite molecular sieve maintained under sorption conditions. It was postulated that the feed contains minute amounts of catalytically deleterious impurities which were sorbed by the catalyst and served as catalyst poisons. Although the precise nature or composition of the catalyst poisons was not known, it was speculated that basic nitrogen compounds, and oxygen and sulfur compounds, may be involved. The substitution of a clay or other sorbent for the zeolite was also suggested as producing some increased activity, but of much lesser magnitude, than is achieved by the zeolite sorbent, although clay was thought to remove a greater fraction of the nitrogen compounds. Sorption conditions included a temperature ranging from 35.degree.-350.degree. F. Patents disclosing such a process include U.S. Pat. Nos. 4,357,232; 4,358,362; and 4,358,363, all assigned to Mobil Oil Corporation.
It has been found, however, that even upon removal of nitrogen compounds from the waxy hydrocarbon stock by solvent stripping or by use of ion exchange resins prior to catalytic dewaxing, the dewaxing catalyst ages in the same manner and, in fact, ages faster than catalysts dewaxing an untreated feed. Thus, it appears that the observed catalyst aging is more complex than any straight forward relationship to nitrogen content. Such is also shown in above-mentioned U.S. Pat. Nos. 4,357,232; 4,358,362; and 4,358,363 where, upon pretreating the feed to remove nitrogen compounds, catalyst behavior is not necessarily altered in a manner consistent with the removal of nitrogen compounds.
It has now been found that even after successive air regenerations of a dewaxing catalyst, catalyst deactivation is irreversible and that the aging rate of the catalyst increases over the fresh catalyst and further increases after each successive air regeneration.
Accordingly, it is an object of the present invention to restore the catalytic activity of an aged dewaxing catalyst to fresh activity, even after multiple regeneration of the aged catalyst.